The invention relates to a locking assembly for securing a rod member in a receiver part connected to a shank for use in spinal or trauma surgery. The invention further relates to a bone anchoring device using such a locking assembly and to a tool for cooperating with such a locking assembly.
U.S. Pat. No. 6,224,598 B1 discloses a threaded plug closure adapted for use in securing a rod member to a bone screw implant, said closure comprising a plug having a threaded cylindrically-shaped outer surface, said plug being received between a pair of arms of a medical implant during use, a central coaxial bore passing entirely through said plug, said central bore having an internal threaded surface which is shaped to receive a set screw. The plug closure and the set screw can be independently installed and the set screw tightened to cooperatively provide capture and locking of the rod in order to secure the rod against translational and rotational movement relative to the bone screw.
US 2003/0100896 A1 discloses a bone anchoring device with a shank and a receiving part connected to it for connecting to a rod. The receiving part has a recess having a U-shaped cross-section for receiving the rod with two open legs and an internal thread on the open legs. A locking assembly is provided comprising a nut member with an external thread which cooperates with the internal thread of the legs and a set screw. The nut member has on one end slits for engagement with a screw tool. The shank has a spherically shaped head which is pivotably held in the receiving part and a pressure element is provided which exerts pressure on the head when the nut member is tightened. By tightening the set screw the rod is fixed in the receiving part. Hence, the rod and the head can be locked independently from each other. The internal thread and the cooperating external thread of the nut member are designed as a flat thread. The implant has a compact design, since an outer ring or nut to prevent splaying of the legs is not necessary.
The outer diameter of the locking assembly is under various aspects determined by the required tightening torque and the thread form. In turn, the overall dimensions of the upper portion of the bone anchoring device are determined by the size of the locking assembly.
Therefore, there is a need for a locking assembly and a bone anchoring device with a locking assembly which has the same reliability as the known devices but which has smaller dimensions of the upper portion. Furthermore, there is a need for a tool for such a locking assembly.